The present invention relates to an original reading device employed in a facsimile system. More particularly, the invention pertains to a close-contact type original reading device.
In general, in a close-contact type original reading device, a plurality of photoelectric conversion elements and a circuit for switchably connected elements in sequence to an output terminal are formed on the same insulating substrate. The length of the photoelectric conversion element array is made equal to the width of an original to be read. The image of the original is transmitted to the array, in an image ratio of 1:1, through an optical fiber array or lens arrangement. In this type of device, the length of the optical path needed for forming an image is reduced, with the result that the size of the original reading device is greatly reduced.
FIG. 1 is a circuit diagram of a conventional close-contact type original reading device. In FIG. 1, reference numeral 10 designates photoelectric conversion elements constructed with a photoconductive film, each element having the equivalent circuit of a photodiode PD and a parallel-connected capacitor. Reference numeral 20 indicates switch elements used for transmitting the output signals of respective ones of the photoelectric conversion elements 10, reference numeral 30 indicates parastic capacitors which are formed by the stray capacitances of leads connecting the photoelectric conversion elements 10 to the switch elements 20 and the input capacitances of the switch elements 20, 40 a charge transmitting circuit, 50 a load resistance, and 60 a bias source. The charge transmitting circuit 40 is made up of charge storage capacitors 41, charge transmitting switch elements 42 and clock lines 43 and 44.
When the image of an original is formed on the photoelectric conversion elements 10, currents representative of the optical intensities of corresponding portions of the image are generated by the photodiodes PD. These currents cause the storage of charges in the parastic capacitors 30. The charges on the parastic capacitors 30 are transmitted to the charge storage capacitors 41 when the switch elements 20 are closed upon supplying a suitable signal to the terminal T1. An alternating voltage is applied to the clock lines 43 and 44 through the terminals T2 and T3 to operate the charge transmitting switch elements 42 successively. The signal charges thus transmitted are shifted (to the right in FIG. 1), and are thus discharged through the load resistor 50, thereby producing an output voltage at the terminal T4. This arrangement may be termed a BBD (Bucket Brigade Device).
The above-described switch elements 20 and charge transmitting switch elements 42 may be MOS transistors manufactured using ordinary integrated circuit techniques. An MOS transistor is suitable as a switch element in an original reading device in that it has a relatively high switching speed and is operable on a relatively low voltage. However, in order to realize the above-described close-contact type original reading device, it is necessary that the photoelectric conversion elements 10 and IC chips including the switch elements 20 and the charge transmitting circuits 40 be mounted on the same substrate and be connected by wire bonding or the like, with the result that the number of connections per original reading device is considerably large. This is a great obstacle to reducing the manufacturing cost of the device and improving its reliability.
The above-described difficulty may be eliminated by employing a technique whereby the switch elements 20 and the charge transmitting circuit 40 are formed using thin film transistors. That is, the switch elements 20 and the charge transmitting circuit 40 are formed using the same process as the photoelectric conversion elements 10. This solves the above-described problem. However, the device so constructed still suffers drawbacks in practical use in that a thin film transistor has a much lower switching speed than an MOS transistor manufactured with integrated circuit techniques. Moreover, it cannot operate on a low voltage. This is, its operating characteristics are still insufficient to make such a transistor fully acceptable as a switch element in an original reading device.
In view of the foregoing, an object of the present invention is to provide an original reading device in which the advantages of both thin film transistors and MOS transistors are effectively realized so that an original reading device is provided which can read an original document at high speed and with high reliability, but which has a simple structure and can be manufactured at a low cost.